Water Surface Overgrowing of the Tatra's Lakes

Ekológia (Bratislava) Vol. 37, No. 1, p. 11–23, 2018 DOI:10.2478/eko-2018-0002 WATER SURFACE OVERGROWING OF THE TATRA’S LAKES JURAJ KAPUSTA, JURAJ HREŠKO, FRANTIŠEK PETROVIČ, DÁVID TOMKO-KRÁLO, JOZEF GALLIK Department of Ecology and Environmental Sciences, Faculty of Natural Sciences CPU in Nitra, Tr. A. Hlinku 1, 949 74 Nitra, Slovak Republic; e-mail: juraj.hresko@ukf.sk Abstract Kapusta J., Hreško J., Petrovič F., Tomko-Králo D., Gallik J.: Water surface overgrowing of the Tatra’s lakes. Ekológia (Bratislava), Vol. 37, No. 1, p. 11–23, 2018. Tatra’s lakes are vulnerable ecosystems and an important element of the alpine landscape. Mainly some shallow lake basins succumb to intense detritus sedimentation, fine fractions of material from the catchment area or to the overgrowing of water level by vegetation. In this paper, changes and dynamics of the 12 Tatra’s lake shorelines that were selected based on the detailed mapping of their extent are pointed out. Changes were assessed by accurate comparisons of historical and current orthophoto maps from the years 1949, 1955 and 2015 – and therefore, based on the oldest and the latest relevant materials. Due to the overgrowing of lakes caused by vegetation, their water surface decreased from -0.9% up to -47.9%, during the examined period. Losses were caused by the overgrowing of open water surface by the communities of sedges and peat bogs. The most significant dynamics of the shorelines during the last decades were reached by those lakes, into which fine sediments were simultaneously deposited by means of mountain water coarse. These sediments made the marginal parts of the lake basins shallower and accelerated rapid expansion of vegetation to the detriment of the open water surface. The overgrowing of shallow moraine lakes lying in the vegetation zone is a significant phenomenon of the High Tatras alpine landscape. It leads to their gradual extinction, turn into peat bogs and wet alpine meadows. Key words: lakes, lake overgrowing, landscape changes, the High Tatra Mts. Introduction The glacial lakes of the High Tatra Mts. represent one of the most important and the most interesting natural elements of the alpine landscape. They are definitely complete and do not pose a natural threat to people today, as is the case in other alpine regions. They greatly in- crease the diversity of the alpine landscape and the whole landscape scenery as well. According to Hreško et al. (2012), alpine lakes are vulnerable, integrated geomorphological-hydrological ecosystems. Their lake basins were finished, especially 27,000 to 19,000 years ago, during the climax of the last Ice Age peaked in the Tatra’s. The last Würm glacia- tion lasted about 60,000 years and ended 10,000 to 8,500 years ago (Lukniš, 1973; Baumgart- Kotarba, Kotarba, 2001; Zasadni, Kłapyta, 2014; Engel et al., 2015). Most lakes were created in the last stages or immediately after the end of the Ice Age. According to Kłapyta et al. 11 (2015), deposition of sediments in lake basins of the Tatra’s lakes began 9,000 to 8,000 years ago. Some shallow moraine lakes, after the appearance of their lake basins, were for certain time probably dry depressions. Their filling with water was only due to the onset of humid climate and consequent positive hydrological balance. It attests to the fact that there are no sediments older than those from the Holocene period in the shallow lakes. The period after the final geomorphological completion of lake basins and the beginning of the sedimentation are considered as the beginning of the Holocene development of the Tatra’s lakes and their basins. Since that time, exogenous geomorphological processes have begun. In many cases, it has strongly influenced the geomorphology of the lake basins and the shape of the shore lines. This development is caused mainly by the transport of sediments into lakes from the surrounding debris cones, rock gutters and walls. As a consequence, some lake basins become shallower by the gradual accumulation of the produced material, so that they gradually lose their open water and finally become extinct. The formation of lake sediments, according to Hreško et al. (2012), is a typical morphodynamic phenomenon of the alpine landscape in the High Tatra Mts. Debris flows and deposition of fine fractions in shallow parts of lake basins are the main causes of the creation of the sediments in lakes. Fine fractions are flooded out from adjacent debris cones, moraines and gutters, especially during their high water saturation due to extreme precipitation (Lukniš, 1973; Gregor, Pacl, 2005; Kapusta et al., 2010; Hreško et al., 2012; Długosz, Kapusta, 2015; Gallik, Bolešová, 2016). Intensive accumulation of weathered material is mainly due to the fact that the lake basins and their surroundings at highly rugged relief by elevation present a local erosive base for geomorphological processes. Debris flows and sediments that are washed out of debris cones and catchment loose movement energy, and deposit intensively as a result of a decrease in their positional energy. Especially in the shallower marginal parts of the lake basins, the sediments accumulate quickly to such extent that they gradually reach the water level and form a typical alluvial plains. These are characteristic geomorphological forms of the relief in the surroundings of many Tatra’s lakes. Their extent might indicate the intensity of sedimentation processes. The abundant extension of these geomorphological forms on the bottom of the valleys indicates the distinctiveness of this phenomenon within the many lakes of the High Tatra Mts. The exogenous geomorphological processes also include organogenic processes, which significantly represent the overgrowing of lakes by vegetation in the alpine region of the Tatra’s. Based on the analyses of the peat sediment thickness (Dyakowska, 1932; Krippel, 1963; Obidowicz, 1996; Łajczak, 2014), it is evident that this process has long-term influence on the development of some lake basins in the alpine environment. From the view-point of the capturing the surface changes of glacial lakes caused by the growth of organic matter or by the overgrowing of open water by vegetation, attention has not been paid to this process. In the case of some lakes, the overgrowing of the lakes’ marginal parts is a significant and typical phenomenon. According to our own measurements and field research, it is an exogenous geomorphological process, which — together with the draining of fine fractions and debris flows — causes the greatest changes and dynamics of the lake shorelines in the condi- tions of the High Tatra Mts. The shape of the shoreline and the rate of change over time can be an important indicator of the postglacial development of the Tatra’s lakes. 12 In this paper, surface changes and changes in the shorelines of 12 selected shallow moraine lakes of the Tatra’s are assessed. Analysis of changes is based on the detailed compari- son of the oldest aerial photos (1949, 1955) and the latest orthophoto maps (2015). Changes in the shoreline due to the overgrowing of their open water by vegetation for the last 60–65 years are depicted. These particular lakes have been chosen according to the apparent overgrowing of the water surface by vegetation. This choice represents Tatra’s lakes where the biggest parts of the water surface have been overgrown. Study area The High Tatras are one of the smallest high mountains in the world with a typically developed alpine relief – with glacial valleys and peaks. It was formed by multiple actions of mountain glaciers during the Pleistocene. The mountains were glaciated at least three times, being the most glaciated mass of the entire Carpathians. The High Tatras lie in an area of only 341 km2, at the borderline of Slovakia and Poland (more than three quarters of their area spread out in the Slovak part). The whole mountain range of the Carpathian Mountains reaches the highest elevation by the peak Gerlachovský štít (2,654.4 m a.s.l.), with another 24 peaks exceeding the 2,500 m a.s.l. The research area is part of the Tatra National Park. Nowadays, there are about 150–230 lakes of various sizes and depths (depending on the size and periodicity criteria) in the High Tatras. Approximately half of them are periodic. In addition, there is a significant number of already extinct, fully overgrown lakes and peat bogs. Mostly, there are small lakes with a surface of less than 1 ha and depth up to 2 m (Gregor, Pacl, 2005). The biggest lakes are Morskie Oko and Wielki Staw Polski (both with a surface of more than 0.3 km2), the deepest being Wielki Staw Polski with a depth of 79.3 m. Fig. 1. The High Tatra Mountains – location of representative lakes: (1) Jamské pleso, (2) Kobylie pleso, (3) Lit- worowy Staw Gąsienicowy, (4) Malé Čierne pleso, (5) Małe Morskie Oko, (6) Mlynické pliesko, (7) Nižné Furkotské pleso, (8) Nižné Rakytovské pliesko, (9) Niżni Toporowy Staw, (10) Trojrohé pleso, (11) Veľké Biele pleso, (12) Wyżni Toporowy Staw. 13 The selected 12 lakes are located predominantly in the lower parts of different valleys, mostly in the forest zone or dwarf pine zone (Fig. 1). They lie at different elevations (from 1,089.0 to 1,734.3 m a.s.l.), as well as the northern and southern side of the mountain. All these lakes are relatively shallow (maximum depth ranges from 0.8 to 5.9 m). The average depth of the water in some lakes is only about 0.5 m. Selected lakes were created either as depression – melted hollows – by melting the dead ice floes buried under the detritus or by the rising of the water level behind waterproof moraine dams.

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